1. Field of the Invention
The invention relates to an oxygen concentration sensor trouble discriminating apparatus for discriminating trouble of an oxygen concentration sensor which detects an oxygen concentration in an exhaust gas of an internal combustion engine.
2. Description of the Related Art
In an internal combustion engine, an air-fuel ratio control is performed by using an oxygen concentration sensor, which is provided in the exhaust system, in order to control an air-fuel ratio of a mixture to be supplied to the engine to a target air-fuel ratio such as a stoichiometric air-fuel ratio. The air-fuel ratio feedback control discriminates an air-fuel ratio of the mixture actually supplied to the engine from an output signal of the oxygen concentration sensor and controls the amount of fuel or air to be supplied to the engine in accordance with the discrimination result.
In the air-fuel ratio feedback control, when trouble occurs in the oxygen concentration sensor, since the air-fuel ratio of the supply mixture cannot be accurately controlled to the target air-fuel ratio, it is important to discriminate the trouble of the oxygen concentration sensor.
Hitherto, to discriminate the trouble of the oxygen concentration sensor, an engine operating state where it is presumed that the air-fuel ratio is rich is detected in operation. The rich operating state is a state where it is detected that the engine lies within a high rotational speed region and, at the same time, a throttle valve enters a high opening degree region. When the rich operating state is detected, if a state where the air-fuel ratio is lean is detected from an output signal of the oxygen concentration sensor, it is stored. After the operation of the engine was stopped, when the engine is again operated, if an operating state in which the air-fuel ratio is presumed to be rich is obtained, in the case where a state in which the air-fuel ratio is similarly lean is detected from the output signal of the oxygen concentration sensor, it is determined that the oxygen concentration sensor is in trouble. When the trouble of the oxygen concentration sensor is determined, an air-fuel ratio control in an open loop is executed due to a fail safe function in place of the air-fuel ratio feedback control.
According to the conventional oxygen concentration sensor trouble detection, however, a fact that the oxygen concentration sensor is in trouble cannot be determined for a period of time from the start of the engine to the stop of the operation. There is, consequently, a problem such that even if the oxygen concentration sensor is actually in trouble, the trouble cannot be decided unless a timing is after the next start of the engine.